1. Field of the Invention
This invention relates to a method and an apparatus for measuring nitrogen in a gas, specifically a method and an apparatus for continuously measuring trace amount of nitrogen in krypton gas or xenon gas.
2. Description of the Prior Art
For example, krypton gas or xenon gas is used in various fields such as the semiconductor manufacturing field or electric device manufacturing field. In particular, in the semiconductor manufacturing field, it has been required to supply a rare gas with a high purity level for a large scale of IC manufacturing.
Purity of gas has a great effect on the product yield of the semiconductor manufacturing process, and thus, is needed to be continuously monitored.
Among the impurities existing in rare gases, nitrogen is used to purge the piping and thus is mixed into the rare gases as an impurity. Moreover, nitrogen is known to decrease the removal ability of a purifier, since it is inactive. Thus, nitrogen is extremely difficult to be removed by a purifier.
In particular, since krypton gas and xenon gas are very expensive, it is getting more and more required that in the analysis process, gas be consumed only in a small amount, and the concentration of trace amounts of nitrogen present in the gas be continuously monitored in real time.
Previously, the trace amounts of nitrogen in krypton gas or xenon gas were measured by GC-MS (gas chromatography-mass spectrometer), which is expensive and can measure only at time intervals. This is a disadvantage, in that, nitrogen concentration cannot be monitored continuously in real time.
Similarly, GC-TCD (thermal conductivity detector-gas chromatography) and GC-PID (photoionization detector-gas chromatography) were used to analyze trace amounts of nitrogen, but they also carry out measurements only at time intervals, and thus is not suitable for monitoring continuously in real time.
Moreover, both analyzers are disadvantageous because the sample gas and carrier gas to be used for GC (gas chromatography) are mixed with each other and thus gas is wasted after measurement. If the sample gas to be measured is a rare gas, such as krypton gas or xenon gas, which is very expensive, running costs increases. And even if the gas is recycled, the purification costs in separating the sample gas from the other gases increases.
As a method for analyzing nitrogen in a gas under atmospheric pressure without using GC, an optical emission analysis method using a glow discharge has been known.
In the above method, discharge is carried out by supplying DC voltage into a discharge tube with a pair of metal electrodes consisting of an anode and cathode, while supplying a sample gas between the electrodes, so that the generated emission of nitrogen is spectrally separated and detected.
However, the glow discharge has inherent disadvantages because a gas cation can collide with the cathode and emit secondary electrons and at the same time decompose the surface of the cathode and emit metal particles; the emitted particles may contaminate the gas, deteriorate the electrodes and cause unstable emission.
In order to solve such problems, a simplified nitrogen analyzer using silent electric discharge, in which metal electrodes are coated with insulating materials, such as glass, and the like, so that the gas and the metal electrodes may not contact each other.
The analyzer supplies high AC voltage to the electrodes enough to maintain a discharge under atmospheric pressure. A sample gas is supplied in a discharge tube at a constant flow rate and is excited by absorbing energy by electron collision or the like. If an energy transition of a gas molecule from high level to low level occurs, an emission of radiant energy, that is, luminescence happens.
As the emission wavelength is specific to the excited gas component, the nitrogen concentration in a gas can be measured by exclusively extracting an emission wavelength of nitrogen using an interference filter and then detecting the intensity thereof.
In particular, the concentration of nitrogen in argon gas, which is used for the above simplified nitrogen analyzer, is measured by separating a wavelength of 337xc2x15 nm or 357xc2x15 nm by using an interference filter and converting the light into an electric signal.
However, as described in the Japanese Patent laid-open Publication Hei 11-326219, since such a simplified nitrogen analyzer needs to use a base gas having a higher ionization potential compared to a gas to be analyzed, it is difficult to measure the nitrogen concentration in krypton gas or xenon gas. And thus, only nitrogen in a rare gas such as argon or helium can be measured.
Therefore, in order to measure the nitrogen concentration in krypton gas or xenon gas using the previous simplified nitrogen analyzer, a GC needs to be installed in the front end part of the analyzer so that the nitrogen in krypton gas or xenon gas is transformed into the nitrogen in argon gas or helium gas. This causes problems in that nitrogen concentration cannot be measured in real time and apparatus costs are increased.
In addition, in the case of measuring concentration of nitrogen in a sample gas using the previous simplified nitrogen analyzer, if an impurity such as oxygen or moisture is contained in the sample gas, nitrogen concentration cannot be accurately measured due to the coexistence effect. Therefore, it is recommended that a purifier for removing the impurity is installed in the sample introduction tube of the analyzer. However, in such a case, the purifier must be replaced before it becomes out of order.
Under such circumstance, the present invention aims to provide a method and an apparatus for analyzing nitrogen in gas, by which nitrogen concentration can be continuously measured with high accuracy and sensitivity without using GC(gas chromatography) and wasting a sample gas, a purifier for avoiding coexistence effect is not required, generation of coexistence effect or plasma abnormality can be determined without damaging the continuously driving condition of an analyzer, and the presence of unknown impurities can be detected.
In order to obtain the above object, the present claim provides a method for measuring nitrogen in a gas, comprising the steps of:
introducing a sample gas into a discharge tube,
collecting an emission light generated by discharge,
extracting a wavelength from the light specific to nitrogen,
introducing said wavelength to a detector, and
continuously measuring the concentration of said nitrogen present in said gas according to the intensity of said light detected by said detector,
wherein, at least one of said wavelengths for measuring the nitrogen concentration is selected from a group consisting of 215xc2x12 nm, 226xc2x12 nm, 238xc2x12 nm, 242xc2x12 nm, 246xc2x11 nm, 256xc2x12 nm, 260xc2x12 nm, 266xc2x12 nm, 271xc2x11 nm, 276xc2x14 nm, 285xc2x12 nm, 294xc2x11 nm, and 300xc2x12 nm.
In addition, the present invention provides the method for measuring nitrogen in a gas, further comprising the steps of:
initially measuring an emission intensity of a gas without any impurities, the impurities being capable of creating a coexistence effect on the nitrogen measurement, as a standard emission intensity.
comparing the measured emission intensity of said sample gas with said standard emission intensity, and
in case said measured emission intensity is smaller than said standard emission intensity beyond a certain extent, determining whether it results from a disorder of the discharge or presence of an impurity or impurities creating a negative coexistence effect.
The present invention further provides an apparatus for measuring nitrogen in a gas for introducing a sample gas into a discharge tube, collecting an emission light generated by discharge, extracting a wavelength specific to nitrogen from the light, introducing said wavelength to a detector, and continuously measuring the concentration of said nitrogen present in said gas according to the intensity of said light detected by said detector,
wherein an interference filter or a spectroscope, which transmits at least one wavelength selected from a group consisting of 215xc2x12 nm, 226xc2x12 nm, 238xc2x12 nm, 242xc2x12 nm, 246xc2x11 nm, 256xc2x12 nm, 260xc2x12 nm, 266xc2x12 nm, 271xc2x11 nm, 276xc2x14 nm, 285xc2x12 nm, 294xc2x11 nm, and 300xc2x12 nm, is installed in front of the detector.
The present invention further provides the apparatus for measuring nitrogen in a gas, further comprising:
means for initially measuring an emission intensity of a gas which does not include any impurities, the impurities being capable of creating a coexistence effect on nitrogen measuring, as a standard emission intensity;
means for comparing the measured emission intensity of said sample gas with said standard emission intensity; and
in case said measured emission intensity is smaller than said standard emission intensity beyond a certain extent, means for determining whether it results from a disorder of the discharge or presence of an impurity or impurities creating a negative coexistence effect.
In particular, the present invention further provides an apparatus and a method for measuring nitrogen in a gas, wherein the sample gas is rare gas and selected from a group consisting of:
krypton gas;
xenon gas;
a mixture of krypton gas and xenon gas;
a mixture of krypton gas and at least argon gas, helium gas and/or neon gas;
a mixture of xenon gas and at least argon, helium and/or neon;
a mixture of xenon gas, krypton gas and at least argon, helium and/or neon.